Turbo-chargers are generally designed to increase the inlet pressure of an internal combustion engine thereby increasing its power and efficiency. In a conventional design a centripetal compressor is driven by a centripetal turbine that is powered by the exhaust gases of the internal combustion engine.
The centripetal compressor of a turbo-charger generally comprises a compressor housing which receives a rotary compressor impeller with radially extending blades. The compressor housing comprises an annular cover, a portion of which closely follows the contours of the impeller blades and a portion of which defines an annular inlet passageway, and a diffuser flange that is fixedly connected between the annular cover and a bearing housing that retains the bearings for the compressor and the turbine. The diffuser may be fixed to the bearing housing by means of set screws or alternatively may be cast integrally with the bearing housing.
There is an ever-increasing demand for turbo-chargers of higher performance particularly with engines of high horse power for heavy duty vehicles. In order to meet this demand it has been necessary to manufacture the compressor impeller from titanium so that the compressor can withstand the high pressure ratios and arduous operating conditions. A disadvantage of an impeller made from titanium or another high density material (e.g. stainless steel) relative to the current aluminum alloy impellers is that the increased density makes the impeller more difficult to contain in the event of its failure. Failure of the compressor impeller may occur through defects in the titanium, consistent use of the turbo-charger at speeds in excess of the top speed limit, or fatigue damage to the material caused by continually cycling between high and low turbo-charger speeds in extreme duty cycles. When the compressor impeller fails in use it is desirable to contain the radially projected fragments within the compressor housing to reduce the potential for damage to the turbo-charger. Generally small fragments are relatively easily contained but larger fragments tend to damage the compressor housing or diffuser flange through their force of impact. At particular risk is the connection between the diffuser flange and the bearing housing. If the two are separated oil leakage from the bearing housing can occur thereby increasing the risk of engine failure.
It is known, for experimental purposes only, or for containment verification tests, to cut a slot in a rear face of the compressor impeller to ensure that when failure occurs it splits into two parts of predictable size and mass. The compressor housing and diffuser flange can then be designed accordingly to ensure containment of the fragmented impeller. However it has still been known for the fragments to pry the compressor housing from the diffuser flange successfully. Attempts to rectify this have included the adoption of a compressor cover manufactured from spheroidal graphite iron. However, this has not proved satisfactory as this material does not absorb as much energy as desired and therefore impact loads transferred to the diffuser flange and bearing housing are greater than normal. Another known approach is to strengthen the diffuser flange in order to improve the chances of containment of the fragments but this has resulted in the impact load of the fragments being transmitted to the set screws connecting the bearing housing and the diffuser flange and caused them to shear or be otherwise torn from the bearing housing. Modifications to the design of the connection between the bearing housing and the diffuser flange to reduce the risk of it being damaged would involve significant changes to the structure of the connection design and therefore significant cost.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.